1. Field of the Invention
The present invention relates to a traction control system for controlling an engine to restrain driving force of a vehicle during excessive or over slippage.
2. Description of Related Art
Automotive vehicles are typically provided with traction control features for preventing aggravation of acceleration due to slippage caused by excessively increased driving torque during acceleration. Traction control is effected by lowering engine output torque or applying braking force to drive wheels upon an occurrence of over slippage of the drive wheels due to excessive driving force so as to achieve a desired amount of slippage of the drive wheels. Such traction control is executed in various modes according to driving conditions and various intentions.
In an attempt to yield the compatibility of driving and acceleration performance regardless of driving loads on a vehicle, the traction control regulates engine output torque more leniently with an increase in driving load. In order to keep acceleration performance during a small size of opening of an acceleration throttle valve, a threshold for a commencement decision of the traction control is changed larger with a decrease in engine output torque. In another control mode, in an attempt to prevent a delay of recovery of engine output torque on a demand of high engine output torque, commencement of throttle valve control is delayed when engine output torque is high. Further, in order for a vehicle to have improved stability of driving and acceleration performance, a drop in engine output torque is reduced according to increases in the coefficient of road surface friction.
Under high engine output torque where acceleration is permitted, if the traction control is executed frequently, acceleration is rather aggravated. If target engine output torque is set lower in vain, a sharp engine output torque drop is produced leading to aggravation of acceleration. Further, a long time of engine output control is undesirable for the traction control in terms of aggravation of acceleration. On the other hand, when slippage develops under relatively low engine output torque, in order for vehicles to be prevented from loosing running stability, the traction control is necessary to be executed. From these points of view, the prior art traction control still involve disadvantages. For example, in the traction control where lenient engine output control is conducted according to driving loads or an engine output torque drop is reduced according to an increase in the coefficient of road surface friction, because of a lack of consideration of driving conditions under which the traction control is executed, the traction control is executed at frequent intervals and for a long time, resulting in aggravation of acceleration. In the traction control where the lower the engine output torque is, the larger the threshold for the commencement decision of the traction control is changed, it is hard to execute promptly the traction control whenever unsteady running occurs. In the traction control where a delay of commencement of the throttle valve control takes place, because there is no consideration about any target of engine output torque in addition to traction control conditions, the traction control suffers significant aggravation of acceleration due to a sharp engine output torque drop.
Vehicles with traction control features often experience acceleration during cornering. In such an event, if acceleration takes place near at a limit in road gripping ability of drive wheels, cornering force acting on the drive wheels becomes less than a lateral acceleration of the vehicle, leading to a pull of, for instance, rear wheels of a rear drive vehicle toward the outside which is so called over steered cornering or a cornering skid. In order to avoid such over steered cornering, one of the prior art traction control regulates engine output torque such that effective drive axle torque agrees approximately with target drive axle torque, which is established according to depressed strokes of an accelerator pedal, so that maximum permissible driving force is not exceeded.
While such the traction control prevents vehicles from being pulled toward the outside during cornering and, consequently, provides the stability of running due to engine output torque not exceeding driving force. However, because of a lack of consideration of variations of cornering characteristics, there are potentially caused slippage and aggravation of acceleration during sharp cornering. Specifically, when sharp cornering takes place on a road with a low surface friction coefficient .mu., vehicle is strikingly unstable, the vehicle gets into a skid with a high probability. In such an event, if front wheels skid, the vehicle undergoes a sudden change of cornering characteristics from a tendency to be slightly under steered to a tendency to be inordinately over steered. On the other hand, if rear wheels skid, there occurs a tendency for the vehicle to encounter inordinate over steering. It is hard to ascertain such a change of cornering characteristics based only on a lateral acceleration, imposing a great burden on the driver. Although it is thought to drop engine output torque impartially during cornering, this however leads to aggravation of acceleration during a negotiation around a slow curve at relatively high speeds.